1. Field of the Invention
The present invention relates to a method and apparatus for producing epitaxial wafers.
2. Related Background Art
It is known in the production of an epitaxial wafer that a semiconductor wafer sliced from a silicon crystal ingot is polished, cleaned and dried, immediately befor its growth process. The semiconductor wafer subjected to such preparatory process is transferred into an epitaxial growth furnace preheated to, for example, 700 to 800.degree. C. and then it is loaded on a suscepter made for example of silicon carbide or high purity carbon and arranged within the furnace. Thereafter, the furnace atmosphere is flushed with a growth atmosphere (hydrogen gas) and then the wafer is gradually heated to a higher temperature to thereby perform a pre-bake and, if necessary, a gas phase etching. During the growth process, a reaction gas containing silicon, e.g., SiH.sub.4 or SiHCl.sub.3 is injected into the interior of the epitaxial growth furnace and thus a silicon epitaxial layer is deposited by reduction or thermal decomposition on the surface of the wafer heated to an elevated temperature of about 1000.degree. C. or over. In this case, the growth time for obtaining the required thickness of the epitaxial layer is adjusted in accordance with the relation between the predetermined growth rate and supply gas concentration. When the epitaxial layer is grown to the target thickness, the supply of the reaction gas is stopped so that the internal temperature of the furnace is gradually decreased and then the epitaxial wafer is removed from the furnace.
In the conventional production process of an epitaxial wafer, the wafer already subjected to the preparatory process is usually handled directly by a robot hand for its transfer into the epitaxial growth furnace. In other words, with the arm of the robot hand being in direct contact with the wafer, the wafer is transferred into the furnace and loaded onto the suscepter from the robot hand within the furnace. In this case, the deposition of dust or dirt or the occurrence of flaw is caused by the contact between the wafer and the robot hand and it is pointed out that such dirt or flaw has the danger of causing the occurrence of such defect as the dislocation of crystal.
Also, during the transfer of the semiconductor wafer, the wafer is only partly supported by the robot hand and therefore it is said that particularly in the case of a large-diameter wafer the occurrence of distortion or deformation is caused in the wafer itself and this also tends to cause an ill effect on the epitaxial growth.
Further, when placing the wafer onto the suscepter within the furnace, the suscepter has already been heated to an elevated temperature and thus the wafer is rapidly heated locally at its portion contacting with the suscepter. Such rapid transfer of heat from the suscepter causes for example a local thermal degeneration of the wafer itself. On the other hand, there is caused nonuniformity in thermal effect between the portion of the wafer contacting with the suscepter and the other portions and thus it is predicted that local variations in temperature distribution tend to cause defects within the wafer and there will be cases where such defects as cracks are caused in the wafer.
Recently, with the silicon semiconductor wafers (CZ wafers) supplied to the market, the tendency is towards increasing the diameter of wafers from those on the order of 200 mm towards on the order of 300 mm and projects are in progress towards realizing the production of 400 mm wafers or ultra large diameter wafers in the near future. Irrspectivee of the fact that such ultra large diameter wafers are over 4 time or so in area as compared with the ordinary wafers of the class which are 200 mm in diameter, the tendency is toward setting their thickness to a little over about 800 .mu.m which is about the same with the ordinary wafers. This is due to the fact that if the ultra large diameter wafers of a thickness which is considerably large as compared with the thickness (about 725 .mu.m) of the ordinary wafers, design changes are inevitably required for the various wafer processes and chip processes as well as the whole production processes of semiconductor devices by the existing equipment and therefore it is difficult to increase the strength of ultra large diameter wafers by increasing their thickness as compared with that of the existing wafers.
Thus, such ultra large diameter wafers are considerably low in strength as compared with the existing wafers and such deformations as strains and deflections due to gravity tend to be caused during their handling. In addition, when heat is externally applied, a longer time is required for the heat to be transmitted to the wafer on the whole so that as compared with the ordinary wafers, it tends to be greatly affected by the localized heating thus causing cracks and defects in the wafer due to the nonuniformheating. Also, since the ultra large diameter wafer is large in surface area and its area which contacts with a support for handling purposes or the like is also increased, the probability of the deposition of dust and dirt and the occurrence of flaws and the like which would have been neglected in the past is increased and they tend to cause further difficulties.